1. Field of the Invention
The present invention relates to an image sensor. More particularly, the invention relates to an image sensor having improved sensitivity and decreased crosstalk, as well as a method of fabricating such an image sensor.
This application claims priority from Korean Patent Application No. 10-2006-0009372 filed on Jan. 31, 2006, the subject matter of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
An image sensor converts optical images into electrical signals. Recent evolution in various consumer products (e.g., digital cameras, camcorders, Personal Communication Systems (PCSs), game equipment, surveillance cameras and medical micro-cameras) has resulted in a sharp increase in the demand for image sensors having improved performance characteristics.
Conventionally, Metal Oxide Semiconductor (MOS) image sensors have been used within a variety of optical imaging products and scanning schemes. MOS image sensors are advantageous because their related signal processing circuits may be integrated on a signal chip so that incorporating products may be miniaturized. MOS processing technologies are also well developed and may be used at reduced cost. Power consumption is also very low, so that the MOS image sensors may be applied to power conscious and battery powered products. Given the excellent resolution and quality performance provided by MOS image sensors, numerous emerging applications are being identified.
However, as the degree of fabrication integration increases for MOS image sensors in order to satisfy demands for finer image resolution capabilities, the size of the constituent photoelectric transducer device, (e.g., a photodiode) in each unit pixel becomes smaller. Unfortunately, reduction in the physical size of the photoelectric transducer device results in a corresponding reduction in the sensitivity of the MOS image sensor.
Furthermore, as pixel density increases in various products, the corresponding inter-pixel distance decreases. This decrease in active device separation leads to crosstalk between adjacent pixels. Inter-pixel crosstalk may be classified into optical crosstalk “A” and electrical crosstalk “B” which are conceptually illustrated in Figure (FIG.) 1.
In optical crosstalk, light incident through a micro-lens and/or a color filter (not shown) is not transmitted to an intended photodiode 4. Instead, the incident light may be transmitted to an adjacent photodiode 4. This result may occur, in part, because incident light 6a is reflected from the top and/or side surfaces of metal wires M1, M2 and M3. In may also occur because incident light 6b is refracted as it passed from the surface of a multilayer structure through inter-layer dielectric films 5a, 5b and 5c which have different refractive indices. This phenomenon is particularly prevalent where the inter-layer dielectric films have non-uniform surfaces.
In electrical crosstalk, an Electron Hole Pair (EHP) formed outside the depletion region of a photoelectric transducer device 2 by long-wavelength incident light 7 is transmitted into an adjacent photodiode 2 through diffusion.
When crosstalk occurs, the resolution is degraded in the case of a gray image sensor to the point where the resulting image is distorted. Furthermore, in the case of a color image sensor (e.g., an image sensor incorporating a Color Filter Array (CFA) of red, green and blue), crosstalk due to red (e.g., relatively long wavelength light) incident light may occur, thereby degrading the resulting image with a tint.
As shown in FIG. 1, a Shallow Trench Isolation (STI) region 3a is typically provided between adjacent photodiodes 4 in the conventional image sensor. STI region 3a may be provided in a p-type doping region 3b and is designed to reduce the possibility of electrical crosstalk. The p-type doping region 3b is formed using an ion implantation process. However, there is a limitation to the depth at which p-type doping regions 3b may be formed under STI region 3a. Thus, it is not possible to satisfactorily provide an electrical crosstalk barrier. Furthermore, the p-type doping region 3b does not provide an effective optical crosstalk barrier.